🌍Geophysics Unit 2 – Seismology

Fundamentals of Seismology

• Seismology studies the generation, propagation, and recording of seismic waves in the Earth and other planetary bodies
• Seismic waves are elastic waves generated by sudden release of energy, such as earthquakes, volcanic eruptions, or artificial explosions
• Seismology provides insights into the Earth's interior structure, composition, and dynamics
• Seismic waves travel through the Earth's interior and can be recorded by seismometers at the surface
• Two main types of seismic waves: body waves (P-waves and S-waves) and surface waves (Rayleigh waves and Love waves)
  • Body waves travel through the Earth's interior
  • Surface waves propagate along the Earth's surface
• Seismology plays a crucial role in understanding Earth's structure, plate tectonics, and earthquake hazard assessment
• Interdisciplinary field that combines principles from physics, mathematics, geology, and computer science

Seismic Waves and Their Propagation

• Seismic waves are elastic waves that propagate through the Earth's interior and along its surface
• P-waves (primary or compressional waves) are the fastest seismic waves and can travel through solids, liquids, and gases
  • P-waves cause particles to oscillate parallel to the direction of wave propagation
  • Velocity of P-waves depends on the elastic properties and density of the medium: $v_p = \sqrt{(K + 4/3\mu) / \rho}$
• S-waves (secondary or shear waves) are slower than P-waves and can only travel through solids
  • S-waves cause particles to oscillate perpendicular to the direction of wave propagation
  • Velocity of S-waves depends on the shear modulus and density of the medium: $v_s = \sqrt{\mu / \rho}$
• Surface waves (Rayleigh and Love waves) are generated by the interaction of body waves with the Earth's surface
  • Rayleigh waves cause particles to move in an elliptical motion in the vertical plane
  • Love waves cause particles to oscillate horizontally, perpendicular to the direction of wave propagation
• Seismic wave propagation is affected by the properties of the medium, such as density, elasticity, and anisotropy
• Seismic waves undergo reflection, refraction, and conversion at interfaces between different materials
• Attenuation of seismic waves occurs due to geometric spreading, intrinsic absorption, and scattering

Earth's Structure and Composition

• Seismology provides crucial information about the Earth's interior structure and composition
• Earth's interior is divided into three main layers: crust, mantle, and core
  • Crust is the outermost layer, with a thickness of 5-70 km (oceanic crust is thinner than continental crust)
  • Mantle extends from the base of the crust to a depth of about 2,900 km
  • Core is the innermost layer, with a radius of about 3,485 km, and is divided into the liquid outer core and solid inner core
• Seismic wave velocities and behavior change with depth, revealing the presence of discontinuities and transitions
  • Mohorovičić discontinuity (Moho) separates the crust and mantle, marked by a sharp increase in seismic wave velocities
  • Gutenberg discontinuity separates the mantle and core, where P-waves slow down, and S-waves cannot propagate through the liquid outer core
• Seismic tomography uses seismic wave travel times to create 3D images of the Earth's interior, revealing heterogeneities and anomalies
• Seismic anisotropy, the directional dependence of seismic wave velocities, provides insights into the Earth's deformation and flow patterns
• Seismological studies contribute to understanding the Earth's composition, including the presence of minerals, phase transitions, and partial melting

Seismometers and Data Collection

• Seismometers are instruments designed to measure and record ground motion caused by seismic waves
• Modern seismometers are highly sensitive and can detect a wide range of frequencies and amplitudes
• Three main types of seismometers: short-period, broadband, and strong-motion
  • Short-period seismometers are sensitive to high frequencies (1-100 Hz) and are used for local and regional studies
  • Broadband seismometers have a wide frequency response (0.01-100 Hz) and are used for global and regional studies
  • Strong-motion seismometers are designed to record large ground motions during strong earthquakes
• Seismometers measure ground motion in three orthogonal components: vertical, north-south, and east-west
• Seismic data is typically digitized and stored in standard formats, such as SEED (Standard for the Exchange of Earthquake Data) or miniSEED
• Global and regional seismic networks, such as the Global Seismographic Network (GSN) and the International Monitoring System (IMS), provide continuous seismic data for research and monitoring purposes
• Ocean bottom seismometers (OBS) are used to collect seismic data in marine environments
• Seismic arrays, consisting of multiple seismometers arranged in a specific geometry, are used for enhanced signal detection and source localization

Earthquake Mechanics and Fault Systems

• Earthquakes occur when stored elastic strain energy is suddenly released due to the rupture of a fault
• Faults are fractures or zones of weakness in the Earth's crust where two blocks of rock move relative to each other
• Three main types of faults: strike-slip, normal, and reverse (or thrust)
  • Strike-slip faults have horizontal motion parallel to the fault strike (e.g., San Andreas Fault)
  • Normal faults have hanging wall moving down relative to the footwall, associated with extensional stress (e.g., Basin and Range Province)
  • Reverse (or thrust) faults have hanging wall moving up relative to the footwall, associated with compressional stress (e.g., Himalayan thrust faults)
• Earthquake focal mechanism describes the orientation and sense of motion of the fault during an earthquake
  • Focal mechanisms can be determined from seismic waveform analysis or first-motion polarities
  • Beach ball diagrams are used to represent focal mechanisms, with compressional and dilatational quadrants
• Earthquake magnitude is a measure of the energy released during an earthquake
  • Various magnitude scales exist, such as local magnitude (ML), body-wave magnitude (mb), surface-wave magnitude (Ms), and moment magnitude (Mw)
  • Moment magnitude (Mw) is the most widely used scale, based on the seismic moment, which is proportional to the fault area and average slip
• Earthquake intensity is a measure of the observed effects of an earthquake at a particular location, often described using the Modified Mercalli Intensity (MMI) scale
• Earthquake recurrence intervals and slip rates can be estimated from paleoseismology and geodetic measurements

Seismic Data Analysis and Interpretation

• Seismic data analysis involves processing, interpreting, and modeling seismic waveforms to extract information about the Earth's structure and seismic sources
• Seismic data processing steps include:
  • Quality control and data formatting
  • Removal of instrument response and conversion to ground motion
  • Filtering and denoising to enhance signal-to-noise ratio
  • Rotation of components and transformation to different coordinate systems
• Seismic phase picking involves identifying the arrival times of different seismic phases (e.g., P, S, surface waves) on seismograms
  • Manual phase picking by trained analysts
  • Automatic phase picking algorithms based on waveform characteristics and statistical methods
• Earthquake location determines the spatial and temporal coordinates of the seismic source
  • Iterative least-squares methods (e.g., Geiger's method) minimize the residuals between observed and predicted travel times
  • Relative location methods (e.g., double-difference) use differential travel times to improve location accuracy
• Seismic tomography uses seismic wave travel times to create 3D images of the Earth's interior velocity structure
  • Travel time tomography inverts arrival time data for velocity perturbations
  • Waveform tomography uses the full waveform information to constrain velocity structure
• Seismic anisotropy analysis investigates the directional dependence of seismic wave velocities
  • Shear-wave splitting analysis measures the polarization and delay time of split shear waves to infer anisotropic properties
• Seismic attenuation studies provide insights into the physical state and composition of the Earth's interior
  • Quality factor (Q) is a measure of the energy loss per cycle due to intrinsic absorption and scattering
• Seismic source studies aim to characterize the properties of the seismic source, such as fault orientation, rupture process, and stress drop
  • Moment tensor inversion determines the best-fitting point source representation of the seismic source
  • Finite-fault inversion resolves the spatial and temporal distribution of slip on the fault plane

Seismic Hazard Assessment and Risk Mitigation

• Seismic hazard assessment quantifies the probability of ground motion exceeding a certain level at a given location and time
• Probabilistic seismic hazard analysis (PSHA) combines information about seismic sources, ground motion prediction equations, and site effects to estimate the likelihood of different ground motion levels
  • Seismic source characterization involves identifying and parameterizing potential seismic sources, such as faults and seismogenic zones
  • Ground motion prediction equations (GMPEs) relate the expected ground motion to earthquake magnitude, distance, and site conditions
  • Logic trees are used to incorporate uncertainties in seismic source parameters and ground motion models
• Deterministic seismic hazard analysis (DSHA) considers specific earthquake scenarios and calculates the resulting ground motion at a site
• Seismic microzonation studies provide detailed maps of local site effects, such as soil amplification and liquefaction susceptibility
• Seismic risk assessment combines seismic hazard with vulnerability and exposure data to estimate the potential consequences of earthquakes, such as damage to buildings and infrastructure, economic losses, and casualties
• Seismic design codes and building standards aim to mitigate seismic risk by ensuring that structures can withstand expected ground motions
  • Performance-based design approaches consider multiple performance objectives and earthquake scenarios
  • Seismic retrofitting techniques are used to strengthen existing buildings and infrastructure to improve their seismic resistance
• Earthquake early warning systems detect the initial P-waves of an earthquake and provide alerts to the public and critical facilities before the damaging S-waves and surface waves arrive
• Public education and preparedness programs raise awareness about seismic hazards and promote actions to reduce vulnerability and increase resilience

Applications in Geophysics and Beyond

• Seismology plays a crucial role in various applications within geophysics and other fields
• Exploration seismology uses seismic methods to image the subsurface for oil, gas, and mineral exploration
  • Reflection seismology maps subsurface layers and structures by analyzing seismic waves reflected from interfaces
  • Refraction seismology investigates the velocity structure of the subsurface using critically refracted seismic waves
  • Seismic attributes and inversion techniques are used to characterize reservoir properties and fluid content
• Seismology contributes to the study of plate tectonics and Earth's dynamics
  • Seismic tomography images the mantle convection patterns and subduction zones
  • Seismic anisotropy provides insights into mantle flow and deformation
  • Seismicity patterns and focal mechanisms help delineate plate boundaries and tectonic regimes
• Seismic monitoring is used for various purposes beyond earthquake detection
  • Monitoring of nuclear explosions and compliance with nuclear test ban treaties
  • Detection and characterization of volcanic activity and eruptions
  • Monitoring of induced seismicity related to human activities, such as fluid injection and reservoir impoundment
• Seismology is applied in geotechnical engineering for site characterization and foundation design
  • Seismic site response analysis assesses the local amplification of ground motion
  • Seismic refraction and surface wave methods are used to determine soil and rock properties
• Planetary seismology investigates the interior structure and seismic activity of other planetary bodies
  • Seismometers have been deployed on the Moon (Apollo missions) and Mars (InSight mission)
  • Seismic data from other planets provide constraints on their internal structure, composition, and evolution
• Seismo-acoustic studies combine seismology with acoustics to investigate the coupling between solid Earth and the atmosphere
  • Detection and characterization of infrasound signals from natural and anthropogenic sources
  • Monitoring of avalanches, landslides, and debris flows using seismic and acoustic methods
• Seismology contributes to the study of Earth's structure and evolution over geological time scales
  • Constraining the formation and differentiation of the Earth's core and mantle
  • Investigating the role of phase transitions and compositional changes in the Earth's interior
  • Studying the seismic evidence for ancient subduction zones and past tectonic events